Cancer is an enormous global health burden, accounting for one in every eight deaths worldwide. A critical problem in cancer management is the local recurrence of disease, which is often a result of incomplete excision of a tumor. Currently, tissue margins must be identified through histological evaluation of the affected tissue. Approximately one in four patients who undergo tumor resection surgery will require re-operation in order to fully excise the malignant tissue. Recent efforts aimed towards significantly reducing the frequency of location recurrence have employed diffuse reflectance, radiofrequency spectroscopy, and targeted fluorescence imaging. However, there remains an urgent need to develop a highly specific and sensitive system and method for detecting tumor margins that will reduce the risk of cancer recurrence and the need for reoperation.
Current techniques for gross anatomic pathology require inspection by a pathologist and are therefore inherently subjective. There exists a need for a system and method that would enable objective analysis of organs samples. It would be advantageous if the system and method could be used to assess a variety of characteristics of a sample including anatomical features, detecting cancerous tumors, and locating tissue margins.
Spectroscopic imaging combines digital imaging and molecular spectroscopy techniques, which can include Raman scattering, fluorescence, photoluminescence, ultraviolet, visible and infrared absorption spectroscopies. When applied to the chemical analysis of materials, spectroscopic imaging is commonly referred to as chemical imaging. Instruments for performing spectroscopic (i.e. chemical) imaging typically comprise an illumination source, image gathering optics, focal plane array imaging detectors and imaging spectrometers.
In general, the sample size determines the choice of image gathering optic. For example, a microscope is typically employed for the analysis of sub micron to millimeter spatial dimension samples. For larger objects, in the range of millimeter to meter dimensions, macro lens optics are appropriate. For samples located within relatively inaccessible environments, flexible fiberscope or rigid borescopes can be employed. For very large scale objects, such as planetary objects, telescopes are appropriate image gathering optics.
For detection of images formed by the various optical systems, two-dimensional, imaging focal plane array (FPA) detectors are typically employed. The choice of FPA detector is governed by the spectroscopic technique employed to characterize the sample of interest. For example, silicon (Si) charge-coupled device (CCD) detectors or complementary metal-oxide-semiconductor (CMOS) detectors are typically employed with visible wavelength fluorescence and Raman spectroscopic imaging systems, while indium gallium arsenide (InGaAs) FPA detectors are typically employed with near-infrared spectroscopic imaging systems.
Conventional spectroscopic devices operate over a limited range of wavelengths due to the operation ranges of the detectors, tunable filters, or other possible system components possible. This enables analysis in the UV, VIS, IR, NIR, short wave infrared (SWIR) mid infrared (MIR), and long wave infrared (LWIR) wavelengths and to some overlapping ranges. These correspond to wavelengths of about 180-380 nm (UV), about 380-700 nm (VIS), about 700-2500 nm (NIR), about 850-1700 nm (SWIR), about 2500-5000 nm (MIR), and about 5000-25000 nm (LWIR).
Spectroscopic imaging of a sample can be implemented by one of several methods. First, a point-source illumination can be provided on the sample to measure the spectra at each point of the illuminated area. Line scanning may also be used where data is generated by illuminating a sample with a laser line. Spectra may also be collected over the an entire area encompassing the sample simultaneously using an electronically tunable optical imaging filter such as an acousto-optic tunable filter (AOTF), a multi-conjugate tunable filter (MCF), or a liquid crystal tunable filter (LCTF). Here, the organic material in such optical filters is actively aligned by applied voltages to produce the desired bandpass and transmission function. The spectra obtained for each pixel of such an image thereby forms a complex data set referred to as a hyperspectral image which contains the intensity values at numerous wavelengths or the wavelength dependence of each pixel element in this image. The method selected to generate spectroscopic data may depend on a variety of factors including the nature of the sample being analyzed, time required for analysis, and cost.
A tunable filter uses birefringent retarders to distribute the light energy of an input light signal over a range of polarization states. The polarization state of light emerging at the output of the tunable filter is caused to vary as a function of wavelength due to differential retardation of orthogonal components of the light, contributed by the birefringent retarders. The tunable filter discriminates for wavelength-specific polarization using a polarizing filter at the output. The polarizing filter passes the light components in the output that are rotationally aligned to the polarizing filter. The tunable filter is tuned by adjusting the birefringence of the retarders so that a specific discrimination wavelength emerges in a plane polarized state, aligned to the output polarizing filter. Other wavelengths that emerge in other polarization states and/or alignments are attenuated.
A highly discriminating spectral filter is possible using a sequence of several birefringent retarders. The thicknesses, birefringences, and relative rotation angles of the retarders are chosen to correspond to the discrimination wavelength. More specifically, the input light signal to the filter becomes separated into orthogonal vector components, parallel to the respective ordinary and extraordinary axes of each birefringent retarder when encountered along the light transmission path through the filter. These separated vector components are differentially retarded due to the birefringence; such differential retardation also amounts to a change in their polarization state. For a plane polarized component at the input to the filter, having a specific rotational alignment at the input to the filter and at specific discrimination wavelengths, the light components that have been divided and subdivided all emerge from the filter in the same polarization state and alignment, namely plane polarized and in alignment with the selection polarizer (i.e., the polarizing filter) at the output.
A filter as described is sometimes termed an interference filter because the components that have been divided and subdivided from the input and interfere positively at the output selection polarizer are the components that are passed. Such filters also are sometimes described with respect to a rotational twist in the plane polarization alignment of the discriminated component between the input and the selection polarizer at the output.
There are several known configurations of spectral filters comprising birefringent retarders, such as the Lyot, Solc and Evans types. Such filters can be constructed with fixed (non-tunable) birefringent crystals for the retarders. A filter with retarders that are tuned in unison permits adjustment of the bandpass wavelength. Tunable retarders can comprise liquid crystals or composite retarder elements each comprising a fixed crystal and an optically aligned liquid crystal.
The thicknesses, birefringences, and rotation angles of the retarders are coordinated such that each retarder contributes part of the necessary change in polarization state to alter the polarization state of the passband wavelength from an input reference angle to an output reference angle. The input reference angle may be, for example, 45° to the ordinary and extraordinary axes of a first retarder in the filter. The output reference angle is the rotational alignment of the polarizing filter (or “selection polarizer”).
A spectral filter may have a comb-shaped transmission characteristic. Increasing or decreasing the birefringence when tuning to select the discrimination wavelength (or passband), stretches or compresses the comb shape of the transmission characteristic along the wavelength coordinate axis.
If the input light is randomly polarized, the portion that is spectrally filtered is limited to the vector components of the input wavelengths that are parallel to one of the two orthogonal polarization components that are present. Only light at the specific wavelength, and at a given reference polarization alignment at the input, can emerge with a polarization angle aligned to the rotational alignment of the selection polarizer at the output. The light energy that is orthogonal to the reference alignment at the input, including light at the passband wavelength, is substantially blocked.
A tunable filter thus passes only one of two orthogonal components of input light. The transmission ratio in the passband is at a maximum for incident light at the input to the tunable filter that is aligned to a reference angle of the tunable filter. Transmission is at minimum for incident light energy at the input is orthogonal to that reference angle. If the input light in the passband is randomly polarized, the best possible transmission ratio in the passband is fifty percent. It is therefore desirable to devise a system and method wherein both orthogonal components of the input light are allowed to transmit through the tunable filter, thereby effectively doubling the throughput at the filter output.